1. Field of the Invention
The present invention relates to an electron gun for use in a color TV or a high definition industrial picture tube, and, more particularly, to a focusing electrode system in an electron gun, in which distortion of an electron beam, which takes place when horizontal and vertical diameters of a main focusing lens are different, is corrected by adjusting power of a dynamic four polar lens.
2. Discussion of the Related Art
Being one of elements in a color cathode ray tube, the electron gun is a device in which three electron beams emitted from cathodes are focused onto a fluorescent screen coated with red, green and blue fluorescent materials inside of the cathode ray tube to generate a fluorescent light, to form a pixel.
FIG. 1 illustrates a longitudinal section of a general in-line type electron gun, and FIG. 2 illustrates a perspective view of the main focusing lens part shown in FIG. 1.
Referring to FIGS. 1 and 2, the electron gun 1 is provided with a triode part 2 for forming electron beams and a main focusing lens part 3 for focusing the electron beams. The triode part 2 is provided with cathodes 4 for emitting thermal electrons, a control electrode 5 for controlling the thermal electrodes, and an accelerating electrode 6 for accelerating the thermal electrons. The main focusing lens part 3 arranged next to the triode part 2 is provided with a focusing electrode system 7 having a first focusing electrode 71 adapted to have applied of a low static voltage, a second focusing electrode 72 adapted to have applied of a high dynamic voltage and having a burring part 723 with upper and lower parts on one end 721 thereof facing the first focusing electrode 71 on a circumference of each of three electron beam pass-through holes 722, and an anode electrode 8 arranged next to the second focusing electrode 72 and adapted to have applied a positive voltage.
Upon the aforementioned electrodes are applied the respective voltages, the electron beams are controlled and accelerated to a preset extent by the controlling electrode 5 and the accelerating electrode 6. Then, the electron beams are elongated in vertical direction by a diverging force in up and down directions of a dynamic four polar lens formed between the first and second focusing electrodes 71 and 72 both by the voltage difference between the first and second focusing electrode 71 and 72 and the burring parts 723. Then, the electron beams are converged by the main focusing lens formed by a voltage difference between the second focusing lens 72 and the anode electrode 8, finally accelerated toward the screen by the positive voltage, and deflected to a certain spot on the screen by a non-uniform magnetic field which is formed by deflection yokes for making a self convergence of the electron beams. The non-uniform magnetic field is adapted to elongate the electron beams in a horizontal direction, forming a haze, a thin dispersion of an image, of the electron beam spot on the screen in up and down directions. However, as explained, since the electron beams have been elongated in the vertical direction already before incident to the main focusing lens by the dynamic four polar lens, the horizontal elongation of the electron beams by the non-uniform magnetic field after incident to the main focusing lens is prevented, to form an electron beam spot of a well rounded circle on the screen. Further, as the size of the main focusing lens is the larger, the main focusing lens between the second focusing electrode 72 and the anode electrode 8 reduces the spherical aberration more, to form the electron beams that spot more clear. In general, the size of the main focusing lens is proportional to sizes of electron beam pass-through holes on opposite ends of the second focusing electrode and the anode electrode.
FIG. 2 illustrates track formed electron beam pass-through holes 725 and 825 formed on the opposite ends, 724 and 824, respectively, of the second focusing electrode 72 and the anode electrode 8 adapted to pass the three electron beams in common for implementing large sized electron beam pass-through holes 725. In order to prevent the main focusing lens from having a horizontal diameter greater than a vertical diameter by shapes like the electron beam pass-through holes 725 and 825, large aperture lens with blade (L-B lens, hereinafter electrostatic field control electrode) 91 and 92 are provided inside of the second focusing electrode 72 and the anode electrode 8. Each of the electrostatic field control electrodes is provided with an electron beam pass-through hole 911 and 921 at a center thereof and blades 912 and 922 of each with a certain width bent at a right angle on both ends thereof. Each of the blades 912 and 922 adapted to be disposed between the three electron beams forms an additional lens so that a horizontal direction converging strength of the main focusing lens is reinforced for preventing the main focusing lens from having a horizontal diameter greater than a vertical diameter, thereby allowing to form three large sized main focusing lenses each of which has a spherical aberration less than the main focusing lens formed by known three electron beam pass-through holes. However, when a diameter of the main focusing lens at the center is compared to a diameter of the outer main focusing lenses, it can be known that virtual effective diameters of the main focusing lenses at the center and at outer portions are different from each other.
FIG. 3 illustrates a graph showing relations between an electron beam diverging angle and an electron beam radius at an outlet of the main focusing lens, from which virtual effective diameters of the main focusing lenses can be calculated according to a known method, wherein the closer the lines to a non-aberration line (the straight line in FIG. 3), the greater the diameter of the lens. The calculation of diameters of the main focusing lenses according to the graph shows that the diameters of the main focusing lens at the center CV and at the outer portions SV in vertical direction are the same 8 mm, and the diameters of the main focusing lens at the center CH and at the outer portions SH in horizontal direction are 7.5 mm and 7 mm respectively, the horizontal diameters SH at the outer portions being smaller than the horizontal diameter CH at the center. This implies that the horizontal diameters SH at the outer portions are influenced by the spherical aberration more than the horizontal diameter CH at the center. In the case when there is no size difference between the horizontal diameters CH and SH of the main focusing lenses at the center and at the outer portions and between the vertical diameters CV and SV of the main focusing lenses at the center and at the outer portions, as shown in FIG. 4A, voltages of the electron beam spot in horizontal and vertical directions measured on the screen when the electron beam is deflected in 3 o'clock or 9 o'clock direction of the screen (hereinafter called "horizontal direction") show no changes in the horizontal direction and an exponential rise in the vertical direction. This is because, as explained before, the horizontal direction voltage comes from the first focusing electrode 71 to which is applied a static voltage that has no changes in voltage and the vertical direction voltage comes from the second focusing electrode 72 to which is applied a dynamic voltage that is changed depending on an amount of deflection of the electron beams to be made. However, as shown in FIG. 4B, in case there is a great size difference between the horizontal diameter SH and vertical diameter SV of the main focusing lens at the outer portions due to the electrostatic field control electrodes 91 and 92, the horizontal direction voltage of the electron beams measured on the screen gradually rises when the electron beams are deflected in the horizontal direction of the screen even if the first focusing electrode 71 is applied the static voltage while the vertical direction voltage rises exponentially. FIG. 4C shows halos in the horizontal direction caused by horizontal over convergencies of the outer electron beams. The halo gives bad effect to the resolution in the periphery of the screen.